* chore: address linter issues

Perform changes suggested by linter; no functional changes.

* error.go: make ErrInvalidID a const

Make ErrInvalidID a constant instead of a variable. This prevent it from
being changed by external packages; a behavior that although allowed by 
the compiler, should probably be considered an invalid operation.

* add benchmark and new failing test for FromString

* fix: let decode look for additional base32 padding

Update FromString and XID.TextUnmarshal so that it looks for discarded
bits in the final source character. This ensures that XIDs that have
been manually tampered with in a way that's ignored by base32 decode,
will not pass as valid.